Conventional ethylene production consists of the following key process operation:
(a) Thermal cracking of hydrocarbons in presence of dilution steam, of C.sub.2 + hydrocarbon at about 15-25 psig and 1,500-1,600.degree. F. to form cracked gas containing ethylene in an amount of 25-40 wt % (with the exception of ethane feed), and other by products such as propylene, acetylene, hydrogen, methane and C.sub.3 + products. The thermal section includes cracked gas cooling, steam generation and C.sub.9 + hydrocarbon condensation. Traces of CO, CO.sub.2 and H.sub.2 S are formed in the cracking. PA1 (b) Cracked gas compression to 400-600 psig, traces of CO.sub.2 and H.sub.2 S removal, drying, and bulk C.sub.4 + product recovery by condensation at about 100.degree. F., using cooling water. PA1 (c) Acetylene conversion to ethylene via selective hydrogenation, chill down and cryogenic recovery of ethylene by fractionation at below -30.degree. F. PA1 (d) Recovery of propylene, propane and C.sub.4 + hydrocarbons by warm distillation at above 80.degree. F. PA1 (e) Cascade refrigeration of ethylene and propylene refrigerants, to support the above, down to a temperature of below -100.degree. F. PA1 (f) Methane refrigeration and or turbo expander to reach refrigeration below -180.degree. F. PA1 (g) In case of Naphtha feed, and to a lesser extent with propane/butane feed, residual liquid products from cracking such as pyrolysis fuel oil and pyrolysis gasoline, which are rich in aromatics, are selectively hydrotreated for di-olefin and olefin saturation. PA1 (h) Benzene/toluene and xylene (BTX) are extracted from the hydrotreated pyrolysis gasoline, and toluene is fractionated and converted to benzene by hydrodealkylation.
Efficient cryogenic recovery of the ethylene is a key element in design of ethylene plants. The motive power for compression and refrigeration, and consequently the capital cost escalates rapidly as the rate of ethylene recovery increases. For example, the typical ethylene recovery of 99.7-99.9% requires much higher investment and 50% more refrigeration energy in the demethanizer as compared with 95% rate of ethylene recovery. Thus, reduction of the marginal refrigeration required for ethylene recovery by using 95% or lower recovery could substantially improve the overall economics of the ethylene plant, if a down stream outlet, other than fuel gas, is found for the 5% more of the unrecovered gaseous ethylene. Normally the unrecovered ethylene 0.1-0.3% is routed with the methane to the fuel gas system. However, the value of ethylene as fuel is only about 15-20% of its equivalent value as downstream product. The ethylene product is commonly used as a feedstock to many downstream processing including ethylbenzene. Production of ethylbenzene from pure ethylene against dilute ethylene feed, although somewhat advantageous from a stand point of the ethylbenzene plant alone, is not an absolute requirement and its relative cost impact is rather marginal as compared with the estimated saving in the ethylene plant.